Direct hydrocarbon fuel cells (DHCFCs) offer an environment-friendly method to produce electricity directly from fossil fuels at a higher efficiency than fuel combustion processes without necessitating the use of a reformer to convert the fuel to hydrogen. Conventional proton exchange membrane (PEM) fuel cells require pure hydrogen fuel to generate electricity by the conduction of hydrogen (H+) ions (i.e., protons) across the electrolyte membrane towards the cathode (oxygen side electrode) where they react with the oxygen to complete the electro-chemical circuit. In contrast to this, DHCFCs operate on the principle of conducting oxygen ions across the electrolyte—in the opposite direction compared to the proton conduction in a PEM fuel cell—and are thus fuel-agnostic since the reaction between the oxygen ions and the fuel actually happens on the fuel electrode (anode) of the fuel cell. Thus, most conventional hydrocarbon fuels can be used directly in a DHCFC without first having to convert (reform) them to pure hydrogen, which is a significant advantage due to obviating the need for an established hydrogen infrastructure to operate DHCFCs. Contemporary DHCFC designs such as Solid Oxide fuel Cells (SOFCs) already exist today, but they operate at high temperatures (T>600° C.), and finding appropriate materials that work in these conditions is a significant challenge. Moreover, these high temperature systems are typically operated in a ‘constantly-on’ fashion in order to reduce the chances of premature failure associated with thermal cycling and the resulting thermal expansion and contraction associated with turning these systems on and off.
Intermediate temperature DHCFCs (operating at T<300° C.) therefore have the potential to improve the implementation of fuel cells for a variety of electrochemical energy conversion applications, from stationary and distributed electricity generation in the range of 10's or 100's of kilowatts (kW) to megawatts (MW), and also for smaller-scale applications such as auxiliary power units (APUs) in the range of <10 KW and for vehicle motive power applications that are typically in the range of about 30 to 80 kW.
An intermediate temperature DHCFC, operating at <300° C., is disclosed in U.S. patent application Ser. No. 14/472,195 (filed Aug. 28, 2014), entitled “Apparatus and Method Associated with Reformer-less Fuel Cell” and this co-pending patent application is hereby incorporated herein by reference in its entirety
However, the performance characteristics of intermediate temperature DHCFCs based on previous designs can be adversely impacted by the limited rate of transport of the ionic active species (i.e. superoxide/oxide/peroxide ion) in the electrolyte. For example, the performance characteristics (current density and specific power) of the DHCFC can be negatively affected by reduced concentration of oxygen in the electrolyte, or by the low rate of diffusion/transport of the oxygen ion through the electrolyte.
Metal-air batteries operate at ambient temperatures and are positioned as energy storage devices with volumetric and gravimetric energy densities in excess of contemporary Li-ion batteries. While redox anodic reactions are typically fast, the energy efficiency of a metal-air battery is limited by the kinetics of the oxygen reduction reaction (ORR). Metal-air batteries are similar to fuel cells in some sense, since they both involve the transport of a mobile oxygen species and both are current limited by the ORR's sluggishness. Also, the analysis of the ORR in fuel cells can be extended to battery systems as well. It is known [Laoire, Mukerjee et al, Journal of Physical Chemistry C, 2009] that the mass-transfer limiting current of the ORR is limited by the solubility of oxygen in the dissolved phase.